css3实现垂直居中-flex

关键兼容代码：

  display: -webkit-box;
  display: -webkit-flex;
  
  display: -moz-box;
  display: -moz-flex;
  
  display: -ms-flexbox;
  
  display: flex;
  
  /* 水平居中*/
  -webkit-box-align: center;
  -moz-box-align: center;
  -ms-flex-pack:center;/* IE 10 */
  
  -webkit-justify-content: center;
  -moz-justify-content: center;
  justify-content: center;/* IE 11+,Firefox 22+,Chrome 29+,Opera 12.1*/
  
  /* 垂直居中 */
  -webkit-box-pack: center;
  -moz-box-pack: center;
  -ms-flex-align:center;/* IE 10 */
  
  -webkit-align-items: center;
  -moz-align-items: center;
  align-items: center;